This invention relates to a disk brake having a piston retraction mechanism using a piston seal.
An ordinary hydraulic disk brake has a piston retraction means having a rubber ring called a piston seal. When the brake pressure is released, the piston seal as the retraction means retracts a brake piston (hereinafter simply "piston") by its elastic restoring force.
As shown in FIG. 1, a piston seal 1 of rubber is received in a seal groove 5 formed in the inner surface of a cylinder 3 defined in a caliper 2 and pressed against the outer periphery of the piston 4 inserted in the cylinder 3.
The edge of the groove 5 on the side remote from the fluid pressure receiving end of the piston (and near to the disk rotor) is chamfered (as shown at 6 in FIG. 1B and FIG. 3).
As the piston 4 is advanced, the piston seal 1 is resiliently deformed at its inner-diameter portion until the gap defined by the chamfer 6 is filled with the seal 11 as shown in FIG. 3B. When the fluid pressure is removed, the piston seal 1 springs back to its original shape, while retracting the piston 4 as shown in FIG. 3C.
This ensures a required clearance between a friction pad 7 (hereinafter simply "pad") and a disk rotor 8 (hereinafter simply "rotor").
If the piston 4 is advanced further from the position shown in FIG. 3B where the pad has deformed to the limit, slip occurs between the seal 1 and the piston 4, thus preventing further deformation of the piston seal 1 regardless of the degree of wear of the pad. This makes it possible to retract the piston under the same conditions at all times.
In the conventional arrangement for piston retraction, assuming that the upper limit of working fluid pressure for the disk brake (that is, the fluid pressure applied by treading a brake pedal when braking) is about 70 kgf/cm.sup.2, the amount of chamfer (W in FIG. 3A) at the edge of the groove 5 is normally set to 0.5-0.8 mm.
However, for vehicles provided with an ABS (antilock brake system), the upper limit of the working fluid pressure is sometimes over 140 kgf/cm.sup.2. In the range above the upper limit, the piston retraction amount may be insufficient, so that the pad may rub against the rotor (which is being rotated), causing what is called "dragging".
In order to prevent such a dragging, according to the conventional concept, it is necessary to increase the amount of chamfer W because the amount of piston retraction depends on the amount W of chamfer and the larger the amount W of chamfer, the larger the deformation of the piston seal and thus the larger the amount of piston retraction.
But this approach had a problem that although the piston retraction will be sufficient to prevent dragging when pressure is released from high pressure, the piston retraction will be excessive when pressure is released from low pressure, so that the fluid consumption increases, thus impairing pedal feeling.
The ideal performance required for the caliper is that the initial loss of fluid amount (which is consumed before the fluid pressure reaches to a predetermined level and which increases the play of brake pedal or ineffective stroke and impairs the pedal feeling) is minimized over a wide fluid pressure range and the dragging torque in a non-braking state is reduced to zero.
In order to meet these requirements, it is required that the relationship between the fluid pressure applied and the piston retraction amount X (when pressure is released) is as near as possible to the line B in the graph of FIG. 4.
The piston retraction amount X should be slightly larger than the minimum required retraction amount A which equals to clearance C (shown in FIG. 5) between the rotor and the pad+caliper deflection D+pad compression strain E and the difference between X and A should be as small as possible, (namely X=B&gt;A and B.apprxeq.A).
With the conventional piston retraction mechanism, the piston retraction at high pressure is insufficient while a sufficient piston retraction is obtained at low fluid pressure, and the piston retraction at low fluid pressure is excessive while a sufficient piston retraction is achieved at high pressure. In other words, as the upper limit of the working fluid pressure increases, it becomes difficult to achieve both good pedal feeling and low dragging torque.
An object of the present invention is to provide a disc brake which makes it possible to reduce the dragging torque while maintaining a good pedal feeling.